There are two types of wire saws used generally and two kinds of manners for cutting a material with the use of such wire saws utilized generally at present.
One of the manners is that a saw wire prepared by bonding or fixing continuously or intermittently abrasive grains or cutting blades onto the surface of a wire element is employed. It is mostly in the form of a loop wherein the opposite ends of a saw wire are joined together. A manner for cutting a material to be worked is such that a looped saw wire is allowed to travel and slide simultaneously with respect to the material to be worked, thereby cutting the same.
However, since an abrasive grain-or cutting blade-fixed wire saw in the above described cutting manner is used for cutting a material to be worked in accordance with a single wire traveling, the material cannot be cutoff simultaneously into a number of pieces, so that its cutting efficiency is poor.
In this respect, a free abrasive grain manner wherein an ingot 43 is cut off by allowing a wire element 41 to slidingly travel (for example, reciprocating traveling or unidirectional traveling) with respect to the ingot 43 as shown in FIGS. 1A and 1B while pouring a processing slurry 44 prepared by kneading abrasive grains with a processing aid into the sliding position is particularly utilized in recent years for cutting off wafers from ingots (materials to be worked) of silicon, a compound semiconductor or the like. Such free abrasive grain manner is also called a multiple wire saw manner, because the operation is carried out in such that a wire element 41 is wound around a plurality of guide rollers 42a, 42b, and 42c with a number of turns, whereby a number of pieces of wafers are sliced from a material in a lump.
In order to elevate slicing performance in a multiple wire saw manner, an inexpensive abrasive grain-bonded saw wire 51a which is prepared by bonding abrasive grains 55 on the surface of a wire element 52a through a binder 54, the cross section of which being shown in FIG. 2A, and another abrasive grain-bonded saw wire 51b which is prepared by bonding abrasive grains 55 on the surface of a covering layer 53 provided on the surface of a wire element 52b through a binder 54, the cross section of which being shown in FIG. 2B have been proposed, respectively. (For example, see Japanese Patent Application Nos. 311187/1996 and 328004/1996)
For the sake of using efficiently the abrasive grain-bonded saw wire 51a or 51b, it is required to afford a tension to the abrasive grain-bonded saw wire 51a or 51b, whereby a contact force (contact pressure) with respect to a material to be worked is elevated in their sliding portion.
However, in the multiple wire saw manner as shown in FIGS. 1A and 1B, although a tension is given to the wire element 41 wound around each of the guide rolls at the opposite ends of the wire element, such tension given to the wire element 41 is not applied merely up to several turns from both of a starting portion of the winding and an ending portion of the winding with respect to each of the guide rollers 42, since the number of windings of the wire element with respect to each of the guide rollers 42 is as much as 50 to 200 times.
For this reason, a major part of the wire element 41 wound around each of the guide rollers 42 (intermediate windings) constitutes a so-called passive state region with respect to a wire tension, so that the wire tension is in an extremely low state. Accordingly, when the abrasive grain-bonded saw wire 51a or 51b shown in FIG. 2A or 2B is applied to a multiple saw wire manner, there has been such a problem that a sufficient cutting ability cannot be achieved by the abrasive grain-bonded saw wire 51a or 51b.
In the above case, although it may be also considered that a number of abrasive grain-bonded saw wires are allowed to travel in parallel to each other, so that a constant tension is afforded to each abrasive grain-bonded saw wire in a cutting section, there is a fear of coming the entire system to be very massive, besides workability for setting up the cutting system becomes worse, because each means and function are required in every single abrasive grain-bonded saw wires for delivering the same, affording tension to the same, traveling the same, and winding the same, and further since the number of saw wires are remarkable (50 to 200). On one hand, even if respective abrasive grain-bonded saw wires are collected at positions before and after stages of a cutting section, and further the respective means and functions for affording tension, traveling and winding the saw wires are processed simultaneously in a lump, delivering means are required with the number corresponding to those of the saw wires by all means.
It is to be noted, however, that the above described passive state region with respect to wire tension seems to contribute to improvements in cutting efficiency, because abrasive grains come to easily get into a gap defined between a traveling wire element and a material to be worked in a free abrasive grain manner (multiple wire saw) which is generally employed at present.